1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to a technique of correcting a curvature of a scanning line in an image forming apparatus for forming an image using a light beam.
2. Description of the Related Art
Conventionally, there is a need for higher image quality, higher speed and higher performance for an image forming apparatus. Particularly, in a laser-scanning image forming apparatus, high-cost parts with high geometric accuracy (a lens, a supporting member, a rotational polygonal member and the like for laser scanning) have been employed so as to maintain a high level of accuracy of an image formation position.
On the other hand, there is also a need for a further reduction in cost of the whole apparatus. To achieve this, it is desirable to reduce the high-cost parts with high geometric accuracy to the extent possible. Therefore, it is necessary to use a digital correction technique, such as, representatively, image processing or laser PWM, to reduce required geometric accuracy, thereby reducing the cost of the apparatus.
Incidentally, a problem with a color printer or the like is how to correct a curvature of a scanning line of a laser. In a typical color printer including a rotational polygonal mirror, an fθ lens and a mirror, a curvature of a scanning line occurs particularly due to a manufacturing error or an attachment error from an optical path of the fθ lens. On the other hand, an inclination of a scanning line occurs due to an inclination of the mirror or an inclination of image carrier.
The curvature or inclination of a scanning line may be corrected by a mechanical method or an image processing method. In the mechanical method, the curvature or inclination of a scanning line is corrected by moving optical parts or the like. However, this method requires additional mechanical parts for moving the optical parts, which is contrary to the need for a reduction in cost of the apparatus.
In the image processing method, the curvature or inclination of a scanning line is corrected by shifting a digital image composed of a plurality of pixels two-dimensionally arranged, in a sub-scanning direction in accordance with curvature points (Japanese Patent Laid-Open Nos. 02-50176, 2003-276235 and 2003-182146). Specifically, it has been proposed that images of a portion of pixels constituting one line are not formed by a corresponding scanning line, but are formed by a scanning line adjacent thereto. A portion of pixels in a pixel sequence forming a first scanning line is replaced by a different portion of pixels in a pixel sequence forming a second scanning line where the different portion of pixels is adjacent to the portion of pixels to be replaced. In other words, replacement of a scanning line is performed. Hereinafter, a deviation and a distortion from an ideal position of a scanning line as well as an inclination of a scanning line are referred to as a curvature.
FIG. 9 shows states before and after correction of a curved scanning line. As can be seen from FIG. 9, a deviation or a distortion from an ideal position can be reduced by correction. However, a bump of one scanning line occurs in a portion to which correction has been applied. The bump can be visually recognized, depending on the image data, resulting in a deterioration in an image.
An image density calculation process (hereinafter referred to as a blend process) may be performed so as to cause the one-scanning line bump to be difficult to see. The blend process is a process of distributing image data corresponding to one scanning line over two scanning lines, thereby causing a bump occurring at a replacement point to be difficult to visually recognize.
However, a color printer that forms an image by superimposition of YMCK may have a problem with such a simple blend process when “horizontal line data having a one-scanning line width of a secondary color obtained by superimposing Y and M” is drawn. Note that Y, M, C and K are abbreviations of yellow, magenta, cyan and black, respectively.
It is assumed that curvature correction in the sub-scanning direction is applied only to the Y color. Specifically, it is assumed that the blend process has been performed with respect to the Y color in two scanning lines adjacent to each other in the sub-scanning direction. In this case, a scanning line of the Y color and a scanning line of the M color that is superimposed thereon have a difference in laser scanning line width (laser spot diameter) corresponding to the ratio of 2 to 1, resulting in a significant color deviation between the Y color and the M color. In particular, a bump caused by correction can be visually recognized in thin line data or character data of a secondary color that have a high contrast. The bump may be significant when the resolution is 600 dpi or less, depending on the accuracy of pixel formation.